Saying that Kasi Lemmons’s new movie, “Black Nativity,” is based on the 1961 Langston Hughes gospel play of the same title is like saying “Finding Nemo” is based on “Moby-Dick”: They’re both fish tales and there the matter ends.

Writer-director Lemmons, who made the much-praised “Eve’s Bayou” in 1997 and the terrific “Talk to Me” with Don Cheadle 10 years later, takes the idea of Hughes’s nativity story — a poetic and deeply spiritualized account of Christ’s birth, one production of which has been staged seasonally in Boston for almost half a century — and shoves it into the far background of a tritely told modern drama about family, faith, and redemption. The music’s great; it’s when the cast stops singing that the problems start.

Still, hell of a cast. (Uh, I mean, heavenly cast.) Relative newcomer Jacob Latimore plays Langston, a street-tough Baltimore teenager whose financially stressed single mom (Jennifer Hudson) ships the boy off to her estranged parents in Harlem for the holiday season. Grandfather is the proud Rev. Cornell Cobbs (Forest Whitaker), whose prized possession is a pocket watch given to him by Dr. King. Grandmother Aretha is a warmly loving soul played by Angela Bassett, who looks fantastic.

From the start, Lemmons doesn’t seem quite sure what she’s making here, a rap musical, an R&B After School Special, a gospel morality play, or all of the above. The early scenes feature Latimore and Hudson singing at each other as hard as they can, and there’s an eerie version of “Sometimes I Feel Like a Motherless Child,” set on a Peter Pan bus, where you wait in vain for all the other passengers to join in.

Once the action moves to Manhattan, “Black Nativity” becomes a sincere, cliched coming-of-age melodrama, with a series of increasingly far-fetched revelations that culminate in a public family showdown at the Reverend’s church. (Surprise: It just happens to be staging Hughes’s play.) By then, we’ve been treated to a dream sequence in which an angelic Mary J. Blige — wings and all — duets with rapper Nas, Tyrese Gibson playing a thug with a secret, and the redoubtable Vondie Curtis-Hall as a pawnbroker who gets the movie’s single most effective scene.

The material’s there for something both churchy and authentically crazy — imagine Tyler Perry off his meds — but the flatfooted dialogue and stock characters slow the drama to a crawl. Whitaker, Bassett, and Hudson do what they can; Latimore doesn’t do enough. (In his defense, I don’t think anyone could breathe life into a line like “This is my Christmas miracle!”) You never doubt that cast and crew want to move you with all their hearts, and when the onstage gospel choir kicks in, you get a brief glimpse of heavenly rewards. But the only actual Langston Hughes comes when Tyrese drops a few lines from “A Dream Deferred” — and by then the movie itself has dried up like a raisin in the sun.